This invention relates generally to methods and apparatus for producing high quality powder particles from molten material. The invention relates specifically to producing metallic powder untilizing the effects of an electric arc.
It is known that an electric arc furnace can be utilized for producing metallic powders. It is known to utilize an ingot of highly purified material such as titanium in an arc chamber or arc furnace in proximity to a graphite, carbon or refractory metal electrode such as tungsten for striking an arc. An arc is struck between the electrode and the ingot both of which are conveniently connected in circuit relationship with an external source of power. The ingot is rotated or spun such that the arc root resides on different portions of the ingot continuously. The arc melts the ingot locally and the spinning effect causes melted particles to be thrown outwardly away from the surface of the rotating ingot after which the particles are quickly solidified into powder. The prior art process is known in the art as the Rotating Electrode Process used under the trademark REP. This process is described on pages 433-437 of the book THE SUPERALLOYS published by John Wiley and Son, New York in 1972 and edited by Chester T. Sims and William C. Hogel. A number of problems and disadvantages are associated with the previously described process and apparatus. Since an ingot of high quality material is necessary initially, the process and apparatus of the prior art require two stages. The preliminary stage comprises casting the high quality ingot and then mounting it in a convenient manner in the chamber where it is to be utilized. This means that mechanical apparatus must be provided for rotating the ingot. This also means that the ingot must be machined to provide a well balanced mass for rotation for powder production. The second stage comprises powder production utilizing the rotating ingot and an electric arc in combination.
Another powder producing apparatus is described in U.S. Pat. No. 3,721,511 issued March 20, 1973 to M. P. Schlienger. In this case a molten pool of material is spun until centrifugal force causes molten material to leave the rotating crucible. A rotating nonconsumable electrode is utilized to heat the melt but not to significantly enter into the operation freeing of molten particles from the melt. Other U.S. patents which do not teach powder production but which do teach arc melting and which are in the name of the latter mentioned inventor are: U.S. Pat. Nos. 3,420,939 issued Jan. 7, 1969, 3,461,214 issued Aug. 12, 1969, 3,649,733 issued Mar. 14, 1972 and 3,651,239 issued Mar. 21, 1972.
In a process sometimes known as the "Centrifugal Shot Casting" process, a spinning melt or pool apparatus is utilized with a consumable electrode to produce powder.
The latter two powder producing apparatus have the disadvantage of utilizing a spinning melt and crucible requiring energy utilization therefor. In addition the arc does not significantly enter into the freeing of the liquid particles.
It would be advantageous if powder producing apparatus and process could be found where the production of an ingot as a preliminary step was not necessary. It would also be advantageous if a process and apparatus could be found where relatively low or medium quality feed stocks such as machine shop chips or scrap parts could be utilized directly in a powder producing process. It would also be advantageous if most of the energy of the process was utilized for producing powder rather than for producing heat for the formation of an initial ingot. It would be further advantageous if material which comes in contact with the molten particles was not reactive, so that relatively pure, high quality powders could be produced. It would also be advantageous if a non-consumable electrode, especially of the arc moving type, could be utilized as part of the novel combination of the apparatus. Such an electrode is described in U.S. Pat. No. 3,793,468 issued to R. R. Akers on Feb. 19, 1974. Another non-consumable electrode is described in U.S. Pat. No. 3,597,519 issued to G. A. Kemeny and R. R. Akers on Aug. 3, 1971. Both of the latter described patents are assigned to the same assignee as the present invention.